This guy gave me his phone number today...

Send him a WhatsApp invitation.

Orrr you can just ring him and speak voice to voice? Is it impossible to have a phone/social life without whatsapp

Just sending him an invite out of the blue might look impersonal :/

yesssss, I hate that.

I was approached by this guy. He was cute and seemed nice. At the end of the conversation (I was at work) he asked for my number. I became really shy and uncomfortable since my work colleagues were surrounding us and I know they were listening in. I told him to write down his number, I did this to make it seem like I wasn't interested in him so my colleagues wouldn't mention it again. He told me not to take his number if I'm not planning to call him.

I've added his number now and I thought he would have what's app, so I could message him on there. However I've discovered that he doesn't have it, I'm quite a shy person as it and I'm thinking to text him instead of calling. What if he doesn't even remember who I am? Do you think this is a good idea?

Yup, if OP wants to build a connection then she needs to have a more personal approach.

I would rather have someone text me first before speaking over the phone. OP doesn't know the guy so texting first would be more viable.

I was approached by this guy. He was cute and seemed nice. At the end of the conversation (I was at work) he asked for my number. I became really shy and uncomfortable since my work colleagues were surrounding us and I know they were listening in. I told him to write down his number, I did this to make it seem like I wasn't interested in him so my colleagues wouldn't mention it again. He told me not to take his number if I'm not planning to call him.

I've added his number now and I thought he would have what's app, so I could message him on there. However I've discovered that he doesn't have it, I'm quite a shy person as it and I'm thinking to text him instead of calling. What if he doesn't even remember who I am? Do you think this is a good idea?