Assuming you've send an SMS to the correct number and you've checked the formatting of the number, then it might be possible that one of the following factors is affecting reception of SMS:
Your phone provider has anti-spam/robocaller features switched on by default
Some operators will now block SMS and calls originating from VoIP numbers. Often this is a setting in your payment plan with the provider - you will need to find this setting and turn it off. For example, T-Mobile offers a product called "Scam Shield" which is quite often switched on by default. https://www.t-mobile.com/customers/scam-shield provides more information.
International or roaming plan required with your phone provider
Its quite likely that without an international minute bundle or roaming plan, your phone will not receive SMS messages from international numbers. Do check this and purchase the respective international plan from your provider.
State operated blocking of VoIP
It could be that your country of residence has an active VoIP firewalling policy in effect. These are state run firewalls that block certain types of traffic from VoIP/cloud based networks, including cloud based phone numbers such as 2ndNumber.
You can still receive SMS text messages sent TO your 2ndNumber. You may find you can also ring out to local regional phone numbers FROM your 2ndNumber WebPhone. But you may find SMS messages sent to a mobile phone carrier that operates in your country are not received.
The following countries are known to block VoIP calls and SMS messages sent to their citizens over the national phone operators:
- North Korea
- Saudi Arabia
- United Arab Emirates
It's worth pointing out that this list can change and most of these measures are a direct result of anti-terrorism legislation in these respective countries. As this legislation changes, so will the list.