> I have a requirement for coder / programmer / software engineer
coder = you tell them exactly what you want in microscopic detail.
software engineer = you tell them what you want to achieve, and leave all the details to them.

Since you're at best "fuzzy" on what you even want, you need a software engineer.

> what is the most appropriate coding language that I should be looking for an individual to have?
That's the wrong question to be asking.

For one thing, sufficiently large problems require implementation in multiple programming languages, so just finding a "foo" programmer wouldn't be that useful.
For another, general purpose programming languages cross many problem domains, so an experienced "foo" programmer might be pretty clueless when it comes to solving the problem in your particular niche.

You need to be looking for people who've had experience in implementing solutions to problems which in some way resemble the problem you're trying to solve.
In other words, you need to be talking to them about their experiences, and their approach to solving problems.
Not taking pot luck with a completed form of check boxes.

> (i.e. between end telecoms device and customer site application)
So one end is a common consumer mobile phone running Android or iOS?