Make sure you consider the battery life on the phone. Bluetooth uses a lot of battery power, I bought the Smartlink to try to be able to use my mobile but having Bluetooth on all day on my phone meant it needed charging in the middle of the day, sometimes more than twice in 24 hours depending on what else I did with the phone during that time. All the battery statistics are without Bluetooth enabled, and the only hints you can get for better battery life are to turn everything off. When I asked at a mobile phone store for a phone I could hear that was a no go, so I asked next if there was a phone I could use with the Bluetooth turned on all day long and the battery last all day and the woman laughed out loud. Short answer, not even close.
And if you want to use the BTooth for incoming calls, many devices have an annoying habit of automatically disconnecting if you have not been using it for a while, so leaving everything ready just in case you receive a call doesn't actually work in practice. My Smartlink trips out all the time, stays connected about half an hour then drops off if I don't make a call.
Another solution to look at is the Sony HBHDS200 - WAY cheaper than Compilots and such, it's a pair of Bluetooth headphones with built in mic so you can use them on a phone call or for music and it rings in your ears for an incoming call, but unlike most bluetooth options you can use any headphones with a standard 3.5mm jack. Use the HBHDS200 and a standard DAI stereo cable and you can get your phone calls and music played into your hearing aids through bluetooth at a fraction of the cost. The mic is on the base part of the HBHDS200 not on the wire, so it doesn't matter what headphone, T-loop or DAI cable you plug into it, equally you can plug it into the audio input cable of an FM system for extra freedom from wires and not to have to juggle back and forth with various leads and plugs, and no need to wear anything around your neck to listen to music or get the NHS to agree to pair the devices. Still have the problem of battery life on the phone, though, you will probably need a portable charger, additional battery or to plug it in a lot.
Sony Ericsson HBH-DS200 Bluetooth Stereo Headset: Amazon.co.uk: Electronics