rules regarding returns never explained

I just took out an Telkom mobile contract at your Crest store Monday. The phone (LG G3) i took is not exactly what i want. I thought we have a 7 day period within which we can decide. This is less than 24 hours I realise its not for me and return it the next day and told i cant because no returns allowed if bought at a store - but this was never explained to me! Had it been explained i would then have no complaints. I simply want to take a better phone, at a higher monthly cost too, its R329 for the one i have versus R399 for the S5 that i wish to take. I have charged and used the phone but it still is new- stickers, etc are still on the phone. Now Telkom is telling me I cannot change the phone when i asked them on Tuesday, only if ordered via the call centre. I feel it is within 24 hours, and this is allowed as no one in the store explained the rules and that there was no 7 day cooling off period or no return policy once i left the store with the phone. I feel if this is taken up i do have valid grounds. Can you please advise and help me to change this handset and move to the higher tariff S5? Kind Regards