A wire will do nothing at all for mobile phones. Stories about 'reflectors' or two
antennas, one inside, one outside, connected by a cable are wasting money.
The only solution - and I did several installation of this - will be a GSM repeater.
But before you invest you have to check two things:
- what system you want to extend: GSM, UMTS, LTE ? You need a repeater that
will handle the transmision mode & frequency of your mobile phone provider.
You did not say in what country you live. Yes, you find some on ebay, but:
- A GSM repeater (or UMTS, LTE) is a receiver with an antenna outside and an
transmitter + antenna inside. And in most countries 'transmitter' need a license
or an approval. Or, in some countries, they simply don't care...
Good luck!