Oldschool solution

OldschoolTypes.js

// In reply to https://gist.github.com/codepo8/9e7dcabb2a9b0c8f3c19441563c6318c
const OldSchoolMobile = {
  keyboard: [' ', '', 'abc', 'def', 'ghi', 'jkl', 'mno', 'pqrs', 'tuv', 'wxyz'],
  decode(sequence) {
    const letters = sequence.split('-').map((presses) => {
      const key = Number(presses[0]);
      const characters = OldSchoolMobile.keyboard[key];
      const repeats = (presses.length - 1) % characters.length;

      return characters[repeats];
    });
    return letters.join('');
  },
  encode(string) {
    const letters = string.split('');
    const presses = letters.map(letter => {
      const key = OldSchoolMobile.keyboard.findIndex(characters => characters.includes(letter));
      return key.toString().repeat(OldSchoolMobile.keyboard[key].indexOf(letter) + 1);
    })
    return presses.join('-');
  }
};

console.log(OldSchoolMobile.decode('44-33-555-555-666')); // hello
console.log(OldSchoolMobile.decode('9-666-777-555-3')); // world
console.log(OldSchoolMobile.decode('9-33-0-2-777-33-0-3-33-888-33-555-666-7-33-777-7777')); // we are developers


console.log(OldSchoolMobile.encode('hello')); // 44-33-555-555-666
console.log(OldSchoolMobile.encode('world')); // 9-666-777-555-3
console.log(OldSchoolMobile.encode('we are developers')); // 9-33-0-2-777-33-0-3-33-888-33-555-666-7-33-777-7777