Simple class to format and convert to E164, mobile phone numbers for the UK and the US. Probably better to just use google libphonenumber.

PhoneNumberFormatter.java

public class PhoneNumberFormatter {

	private final String simCountryIso;

	private final Map<String, String> countryCodeMap = Collections.unmodifiableMap(
		new HashMap<String, String>() {{
			put("GB", "44");
			put("US", "1");
		}});

	private final Map<String, String> countryPatternMap = Collections.unmodifiableMap(
		new HashMap<String, String>() {{
			put("GB", "^(\\+44\\s?7\\d{9}|\\(?07\\d{9}\\)?)");
			put("US", "^([\\+][1]{1})?\\(?([2-9]{3})\\)?([0-9]{3})([0-9]{4})$");
		}});

	public static PhoneNumberFormatter init(Context context) {
		final TelephonyManager telephonyManager = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
		return new PhoneNumberFormatter(telephonyManager.getSimCountryIso().toUpperCase());
	}

	public PhoneNumberFormatter(String simCountryIso) {
		this.simCountryIso = simCountryIso;
	}

	public Optional<String> getFormattedPhoneNumber(String phoneNumber) {
		if (phoneNumber == null || simCountryIso == null) {
			return Optional.absent();
		}

		phoneNumber = phoneNumber.replaceAll("\\s+", "");

		if (!isValid(phoneNumber, simCountryIso)) {
			return Optional.absent();
		}

		if (!countryCodeMap.containsKey(simCountryIso)) {
			return Optional.absent();
		}

		final String dialingCode = countryCodeMap.get(simCountryIso);
		final boolean startsWithCountryCode = phoneNumber.startsWith("+" + dialingCode);
		final boolean startsWithZero = phoneNumber.startsWith("0");

		if (!startsWithCountryCode && !startsWithZero) {
			final String formattedString = "+" + dialingCode + phoneNumber;
			return Optional.of(formattedString);

		} else if (!startsWithCountryCode) {
			final String numberWithoutFirstDigit = phoneNumber.substring(1, phoneNumber.length());
			final String formattedString = "+" + dialingCode + numberWithoutFirstDigit;
			return Optional.of(formattedString);
		}

		if (!phoneNumber.startsWith("+")) {
			phoneNumber = "+" + phoneNumber;
		}

		return Optional.of(phoneNumber);
	}

	private boolean isValid(String phoneNumber, String simCountryIso) {
		if (!countryPatternMap.containsKey(simCountryIso)) {
			return false;
		}
		final String pattern = countryPatternMap.get(simCountryIso);
		return Pattern.compile(pattern).matcher(phoneNumber).matches();
	}

}

public class PhoneNumberFormatterTest extends BaseTest {

	private PhoneNumberFormatter phoneNumberValidator_GB;

	private PhoneNumberFormatter phoneNumberValidator_US;

	@Before
	public void onSetup() {
		phoneNumberValidator_GB = new PhoneNumberFormatter("GB");
		phoneNumberValidator_US = new PhoneNumberFormatter("US");
	}

	@Test
	public void test_phone_number_validation_isCorrect_GB() {
		final String startPhoneNumber = "07000000000";
		final String expectedPhoneNumber = "+447000000000";
		final Optional<String> formattedPhoneNumber = phoneNumberValidator_GB.getFormattedPhoneNumber(startPhoneNumber);
		assertTrue(formattedPhoneNumber.isPresent());
		assertEquals(expectedPhoneNumber, formattedPhoneNumber.get());
	}

	@Test
	public void test_phone_number_with_code_validation_isCorrect_GB() {
		final String expectedPhoneNumber = "+447000000000";
		final Optional<String> formattedPhoneNumber = phoneNumberValidator_GB.getFormattedPhoneNumber(expectedPhoneNumber);
		assertTrue(formattedPhoneNumber.isPresent());
		assertEquals(expectedPhoneNumber, formattedPhoneNumber.get());
	}

	@Test
	public void test_invalid_phone_number_doesFail_GB() {
		final String invalidPhoneNumber = "+10700000";
		final Optional<String> formattedPhoneNumber = phoneNumberValidator_GB.getFormattedPhoneNumber(invalidPhoneNumber);
		assertTrue(!formattedPhoneNumber.isPresent());
	}

	@Test
	public void test_phone_number_validation_isCorrect_US() {
		final String startPhoneNumber = "2345678900";
		final String expectedPhoneNumber = "+12345678900";
		final Optional<String> formattedPhoneNumber = phoneNumberValidator_US.getFormattedPhoneNumber(startPhoneNumber);
		assertTrue(formattedPhoneNumber.isPresent());
		assertEquals(expectedPhoneNumber, formattedPhoneNumber.get());
	}

	@Test
	public void test_phone_number_with_code_validation_isCorrect_US() {
		final String expectedPhoneNumber = "+12345678900";
		final Optional<String> formattedPhoneNumber = phoneNumberValidator_US.getFormattedPhoneNumber(expectedPhoneNumber);
		assertTrue(formattedPhoneNumber.isPresent());
		assertEquals(expectedPhoneNumber, formattedPhoneNumber.get());
	}

	@Test
	public void test_invalid_phone_number_doesFail_US() {
		final String invalidPhoneNumber = "23456789";
		final Optional<String> formattedPhoneNumber = phoneNumberValidator_US.getFormattedPhoneNumber(invalidPhoneNumber);
		assertTrue(!formattedPhoneNumber.isPresent());
	}

}