Call recorders can be very cheap and you can get them from Maplins.
The best one is the Truecall. It is pricey but will record your calls, screen your calls, reject unwanted callers etc.
Follow this link to read the review of the Truecall telephone recorder and call manager.
This is the Rolls Royce of call recorders.
It's a bit pricey
You can also buy cheaper devices from Maplins. Truecall is the best - but the important thing is to record your calls.
A TP8 from Amazon and a digital recorder is also very good
You can easily record calls on your Android mobile phone by installing a free automatic call recorder from the Play Store. If you have an iPhone you can use an Olympus TP8 and a digital recorder.