I reboot my 700wx about 8-10 times per day. I've had it lock up in calls, receiving call, trying to make calls, while trying to receive a call while in a data session, and all variations on that theme.
When it works, it's great, for about 10-20 minutes, then it stops working again.
FYI, it's not the phone. These phones are also computers. It is 3rd party software on your phone that is not syncing up and interfereing with your phone's operating system.
When your phone started doing this, had your recently added 3rd party software?
Try doing a sync on your phone with your PC. Then do a hard reboot of your phone. Caution: Make sure you have your phone numbers are backed up in Outlook on your PC because your phone sim card will be wiped clean. It's the same as starting from day one with your phone.
Then resync your phone--it may take a little longer if it has to reload all your numbers.
Try not to reload your 3rd party software. Your phone should work fine, now. Add back the 3rd party software one at a time and use your phone for a couple of days, checking for interference. As you add each 3rd party software, at some point you will notice your problems return. Sometimes, it can be the combination of one or more 3rd party software programs.
It most likely is not your phone--just software you have added. Many shareware programs (free) do not have all the bugs worked out. You become the beta tester--unknowingly. All it takes is one weak or bad line of code to cause your grief. Many of the people who develop some of these 3rd party software programs are garage programmers. This is not to say that some are not good, but it's just a risk you take. I have several programs that have been written by some sharp 13 and 14 years olds. However, if their software taps into other resouces for input, and they don't have a licensing agreement, at some point it may not act right and lock up your phone.
Normally, the programs you have to buy, are more reliable. However, there can still be conflicts if if is not licensed by your O/S provider.
These problems can happen on phone or pc's with any operating system. MAC's and Apple's are just about immune. The reasons are that they are more proprietary, and there are fewer novice programmers out there to mess them up and also fewer hackers. The new apple phone will probably be very reliable--but costly.