I have got a contract with True, rooming etc. and want to keep my number so I do not want to change to DTAC or 12Call but my question is if more people have problems with their True mobile connection in the On Nut area? Can I keep same number but change provider?

Can I keep same number but change provider?

Yes, it's called Mobile Number Portability (MNP).

See http://truemove.com/mnp/en/know_mnp.html for info. Of course, that web page is True's and intended for users changing *to* True, but the process is essentially same to change from True to another provider.